The invention relates to the sensing of the level of a material, particularly liquids.
Although the invention may be used in many types of applications, the invention is disclosed herein for use in controlling or indicating the level of flammable liquids during the filling operations of tank trucks for purpose of explanation. When tank trucks used for hauling flammable liquids are filled, it is advantageous from a safety standpoint as well as a control standpoint to sense the level of the liquid. Overfilling of such a vehicle or container involves not only waste but the danger of the liquid exploding due to sparks, static electricity or other forms of ignition. It would, of course, be possible for the operator of the filling pumps to make a visual survey of the liquid level of the tank and when the level has reached a predetermined height to terminate the filling operation. However, flammable fluid handling vehicles are often compartmentalized making visual observation of a plurality of tanks impossible or at least very difficult. Also, there may not be the necessary personnel in attendence to supervise the filling operation.
It thus becomes apparent that some form of automatic liquid level control or detection is necessary to supervise the filling operation. The system must also be reliable in view of the grave consequences which could result from the failure of the supervisory system. Therefore, the system should be made as fail-safe as possible.
Various systems have been devised to automatically sense and control the level of material within a container. These systems utilize a wide variety of sensors including capacitive type sensors, contacts, magnetically and mechanically operated switches such as float operated switches, various types of thermal devices and optical sensors. As these systems developed, it became apparent that they could fail allowing the containers to overflow. To solve this problem, various types of fail-safe devices were proposed.
One such device relies on the detection of pulses to control the level of the material being sensed. If the material reached a predetermined level or if any component in the system failed, the sensing signal became continuous instead of pulsating. The continuous signal was sensed to terminate the filling operation or provide the proper indication. Generally, these systems were not sufficiently fail-safe, were expensive to assemble and manufacture and were not reliable.